Priority is claimed to Japanese Patent Application Number JP2004-162683 filed on Jun. 1, 2004, the disclosure of which is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to a treatment apparatus for removing fluorine content from water to be treated and a method of treating water to be treated using the treatment apparatus.
2. Description of the Related Art
Currently, the reduction of industrial wastes and the separating and recycling of industrial wastes are important subjects from an ecological standpoint and are urgent issues for companies. The industrial wastes include various fluids containing substances to be removed.
The fluids mentioned above are expressed as various terms, such as the terms “sewage”, “waste water”, and “liquid waste”. Fluid, such as water or chemicals, containing a substance to be removed will be hereinafter called “waste water” and described. The waste water may be recycled into a clean fluid for reuse by removing the substance to be removed from the waste water, such as by using a costly filtration apparatus to filter out the substance to be removed from the waste water. Moreover, the substance to be removed, which is separated from the waste water, may undergo treatment for industrial waste. Water, in particular, is made clean by filtration to meet environmental quality standards so that the water is released into nature, such as rivers or the sea, or is recycled for reuse.
In the course of a process for manufacturing a semiconductor device, large quantities of waste water are produced. In an etching process of the manufacturing process, waste water having fluorine content such as hydrofluoric acid is discharged. It is known that the discharge of waste water containing a high concentration of fluorine into nature causes a disturbance in ecological balance. The removal of fluorine content from waste water is therefore of extreme industrial importance.
The conditions of the release of waste water having fluorine content are set, based on a reference value prescribed by, for example, the Water Pollution Prevention Law or municipal laws. Specifically, the concentration of fluorine contained in waste water must be equal to or less than 8 mg/L. Moreover, regulations may be carried out on the total amount of fluorine discharged.
There are proposed various techniques for removing fluorine contained as mentioned above. There is proposed a technique for removing fluorine contained in waste water, which involves using different treatment tanks for biological treatment and chemical treatment. This technique is described for instance in Japanese Patent Application Publication No. 2001-54792. Another technique for removing fluorine content involves preparing a plurality of reaction tanks; then adding calcium to raw water contained in one of the reaction tanks, thereby forming a seed agent containing a substance in sol form; and adding the seed agent to raw water contained in the other or another tank, thereby treating fluorine content. This technique is described for instance in Japanese Patent Application Publication No. Hei 06-312190. The technique for removing fluorine further includes employing a coagulating sedimentation, which involves using a polymer flocculant to coagulate fluorine contained in waste water, thereby extracting sludge from the waste water.
However, a method of treating waste water having fluorine content, as disclosed in the publication No. 2001-54792, has to include a plurality of steps of treating waste water. Thus, the method has the problem of raising costs due to large-scale facilities required for these steps. The method further has the problem of having difficulty in achieving stable waste water treatment because of using organisms to treat organic content.
A method of treating waste water, as disclosed in the publication No. Hei 06-312190, has the problem of having to include an additional step of treatment such as coagulation in order to solidify fluorine content derived from waste water, because the waste water contains an extremely low concentration of fluorine.
The technique for removing fluorine contained in waste water, employing the coagulating sedimentation using a commercially available polymer flocculant, has the problem of large quantities of sludge being extracted from the waste water and resulting in industrial wastes. The technique further has the problem of rendering it difficult to recycle the sludge, because the sludge has large quantities of flocculant mixed therein.